2000 silverado and ultrastart 1272

I'm surprised you are having trouble with that truck.  You are using tried and true componants on a very straight foreward vehicle.  Just so we don't overlook anything, here is the wiring :

U1272

6 Pin Ignition Harness

1 Yellow  (+)  Starter Output      Yellow2 Green (+)    Accessory Output            Orange3 Red (+)       +12V constant      Red4 Red (+)       +12V constant      RED / White5 White (+)    Flex Relay Output              White  ***Programmed for IGN2 - Default6 Blue (+)      Ignition Output      Pink

2 Pin Harness

Black (-) Main Chassis Ground Chassis GroundWhite     Selectable Parking Light Output         Brown @ BCM   ***Jumper set to +

9 Pin Harness

1 Yellow (-) Rearm Output               Not Used2 Brown (-) Disarm Output              Not Used3 Black (-) AUX1 Output                 Not Used4 RED / White (-)     Trunk Release   Not Used5 WHITE/ Blue (-)    Horn Output        Black @ BCM6 Pink (+) Brake Input         White @ Brake Pedal7 GREEN / WHITE (-)  Hood Pin Input            Hood Pin Switch8 Blue/White (AC) Tach Input      White @ Instrument Cluster9 Blue (+/-)   Glow Plug/WTS/Trigger In     Not Used

Lock Harness

Green (-) Lock to relay to convert to (+)        Light Blue @ BCM      ** Can also use DEI 451MBlue (-)   Unlock          to relay to convert to (+)         White @ BCM **   to convert signals.

3 Pin Bypass Harness

Red      +12V    Not UsedBlack       Ground Not UsedWHITE/ Violet     (-) GWR           Not Used

Fortin INT-SL+

D2D to U1272Pink Ignition Input to U1272 thick Blue Ignition1 Output @ 6 Pin connectorPurple to OBD2 Pin 2  ( This wire is usually Violet )

A few thoughts...

You are still listing the GWR connection between the U1272 and INT-SL+.  This is not needed with D2D and should be disconnected.

Might be a good idea to start programming from ground zero. 

1. Perform Factory Reset on U1272.2. Set U1272 Progrmming Options.  All defaults should be good, but change Bypass to Fortin.3. Program the INT-SL+ ( No GWR connection )  Note that when the engine is started, the U1272 will confirm the    automatic Tach Learn.

If that fails, try another INT-SL+.

If that fails, try another U1272.If that fails, go W2W using the U1272's 3 Pin bypass connector and wiring as per the INT-SL+ Without DataLink Plan2 instructions.

Soldering is fun!

Thanks, I'll triple check all that. I did do the factory reset, and the bypass set to Fortin. One thin though, I did do a W2W of sorts. I cut the blue and white wires on the D2D harness, and plugged the the harness in. I did connect the WHITE/ violet to GWR. However, I did not connect the black and red on the same harness as the white.violet, as I thought when I plug the harness into the Fortin for programming, the LED is lit, showing it gets power. Should I still connect the red and black on that 3-pin harness?

I use them because it's easier with all three wires going to the same plug, but how you have it is OK.  What you did is actually W2W with the Data wires cut.  The next step would be connecting the Fortin's Red & Black wires to that bypass connector or even this -  Red to Thick Red ( on 6 Pin plug ), Black to semi-thick Black ( on the Parking light harness ) and Blue to the WHITE/ Violet ( same ).  Might not help too much if the INT-SL+ is getting power ( LED working ).  And leave the GWR connected now that the D2D Blue & White wires are cut.Soldering is fun!

Happy New Year everyone. Hope everyone starts the new year on a positive note. As for me, this straight forward install is a no go, I have run every diagnostic test I can think of, and this thing will not start. I think either the 1272 is bad or the INT-SL+. I'm leaning towards the INT-SL+. It will not program. I am out of ideas, and at wit's end

Brown (-) Disarm Output from the U1272 should go to the light green wire on the purple plug of the BCM. That's why the alarm is going off when you unlock the doors. Double check the harness going into the INT-SL as it can be accidentally be inserted upside down. Also make sure it's pushed all the way in. I've had issues with it coming apart easily if not pushed in tight. If you are able to start the vehicle with they key and then have the remote starter take over, it's most likely the INT-SL acting up or not connected properly. Something just came to mind also looking thru the INT-SL install guide that states "In Data-Link mode the analog Input (unlock, ground out) are disabled for security.." You stated you are running W2W not D2D i believe. When you are programming the INT-SL+, after Step 4, DON'T press any buttons on the Ultra Start remote and just start the truck. Don't even wait to see if the LED flashes on Step 7.   Before programming anything, disconnect the GWR wire from the INT-SL and don't connect it back until the programming is complete. Also, some GMs BCM will go to sleep mode after a while. When you are ready to program everything again, close and open the driver's door and hit unlock on the factory remote to ensure the BCM is awake. This may be preventing the purple data wire at the OBD II from sending info. Then proceed to attempt programming again. Unplug everything from the INT-SL, then hold the button down and plug in Connector 1 and once LED turns on, release the button. Plug in Connector 2 and then start the truck with the key. What does the INT-SL do after that? Reconnect the GWR wire to the INT-SL+ Try remote starting the truck.

Finally was able to get to work on the truck. Connected the brown wire to disarm, followed all directions in above posts, and still nothing. If I keep the key in the ignition, truck turned off, still nothing. If I start the truck, and while running, try to remote start it, the lights flash and still nothing. I believe I will try to exchange the Fortin, and see if that is not the problem

If you start the truck with the key and then press the remote start button and it does NOT takeover, it's not an issue with the Fortin module. The module only comes into play when you are initially starting the truck. If pitstop mode doesn't work, (starting first with the key, then hitting remote starter to takeover), it sounds like either a tach issue or a wiring issue at the ignition harness. Or worse case, a bad remote starter brain not engaging all the relays. Also, not sure if mentioned prior, but a bad ground also causes funky stuff like that.

I checked the ground, seems good. When programming the 1272, I plug all harnesses in, and then install both fuses. I then put my foot on brake and start truck. Then I push the program button on the antenna, then push again for 5 seconds. When I push on it the second time, the horn honks twice and the lights blink twice immediately.

That sounds like you are doing a system reset. After the reset, you have to then start the vehicle again so it can learn the tach. It doesn't seem like it's learning tach. I had an issue with a 2008 Cobalt where an Ultrastart G11 series didn't learn the tach signal since the voltage was very low coming from the injectors and coils. I switched to a Crimestopper unit and picked it up right away. Not suggesting you switch brands as Ultra Start is a very well known and reliable brand but it's sounding like a tach issue at this point. Especially since it won't keep the truck running in pitstop mode. Correction - Sorry about that, I don't normally install with Ultra Start and confused the system reset procedure and the tach learn procedure.

So are you saying after I do the tach learn procedure, shut the truck off and start again?