'99 Tahoe wont run

I have a '99 Tahoe that had a DEI Model 555L(I think?) alarm.  When I bought the truck the dealer of course didnt have the key fobs and the alarm didnt work.  I installed a new stereo head unit and decided to cut the giant mess of wires and the old alarm out.  Now the truck will start but stop immediatly.  Im guessing that it has something to do with the alarm and how it was wired.  What wires do I need to look for and how do I fix 'em

The 555L is not in itself an alarm or a remote start.... It is a module that lets the remote start defeat the car's PASS-Lock II anti-theft system. Part of the installation is that a small yellow wire coming from the steering column is cut in half, and each half is connected to a different wire from the 555L. So, the solution is to find the yellow wire that's cut in half, and put it back together. Do as nice of a job as you can, so you won't get stuck again.

I traced the wires from that module to the small yellow wire you mentioned.  I had to remove the panel at the bottom of the steering column.  Sure enough it was cut in half and two wires were spliced in.  I soldered the yellow wire back together and the truck starts and run great now.  Thanks!