1994 Chevy C1500 Truck Brake Light Mystery

I just went to the junk yard and got a new door switch for the courtesy lights. I plugged it in and immediately the "key in ignition" bell started sounding off. This is rich.    I also grabbed a spring from the turn signal innards that, broken, was keeping the signal from defeating after turning the wheel. I cleaned and relubed all the contacts. After both these fixes, I held my breath and reinserted the CTSY fuse, and nothing was remedied. The CTSY circuit still has a short somewhere that is energizing the HMSL relay which in turn is activating my brake lights!   And fixing the CTSY door switch has now remedied one of my least favorite alarms... the "hey your keys are in your ignition" bell.

that truck does not use a relay for the brakes. at least it's not setup that way from the factory.

If it does not use a relay for the brakes, can you theorize why it would use a relay for the HMSL? The 'third' brake light would seem, to me, to draw even less amperage. I've been told by a few people now that the relay as I've described it is an HSML relay. Yet it is affecting the funtion of the brakes.     This is an image of the relay.   Two yellow wires, a black, and a white. It is mounted above the brake master cyclinder on the firewall. It is stamped        Omron 12077867 Z7365

Sorry I'm missing something, what's an HMSL relay?

OK, I've seen the photo, next stage would be to trace this relay's wiring!  Can you not get hold of a Chilton manual for that year from your local library?  I would think about rewiring the brake lights from the brake pedal switch back avoiding that relay.  What's the link to the dome (courtesy) lights?

I wish I knew what the link was to the courtesy lights. The Chilton and Clymer manuals are no help. There is no mention of HMSL in either of them. There are five wires leading back to the rear bumper on my truck and only four of them are leading to the taillight assembly harness. A yellow one is loose and terminated.   It used to be connected to the HMSL (high mount stop lamp) on the topper. The topper installer had connected the HMSL to this yellow and tapped into a black wire behind the left taillight assembly. I have since removed this HMSL wiring that the topper installer used. The clymers diagam I have shows yellow going to the brake lights. But the plug has not been altered in any way. It is a four pin plug on the harness going to the taillights, and there are five wires leading out of the split loom plastic conduit leading to the back of the truck. I'm really starting to suspect that this yellow got shorted somehow in the messy wiring that the topper installer had going up to the HMSL on the topper.

Not only would  I reroute the brake pedal switch back to the back lights, I 'm suspicious of the yellow being taped to the black. Now I don't know the vehicle and I'm 4,500 miles away! ..But black is normally a ground on US, old UK colours and some Japanese. If the yellow came from the top light, check what the black did. 5 wires seems too few because of rear fog lamp(s) and separate indicators to brake lights (amber, look at any Euro or Japanese, they all have separate amber flashers, our equivalent would be at least 8 wires also and I think this also applies to N. America, sidelights are separated (ie fused, either left right or diagonally) as a safety precaution.  Next disconnect  that darn relay and then test drive the vehicle, see if anything isn't working apart from what we already know about.  I don't know how relevant this is because big trucks are a rarity here but I've worked a few Ford Litenings and they have a courtesy light on the outside rear of the cab to illuminate the load area. Could there have been one on yours or something similar that's been disconnected and has a lose cable that's shorting out?

Ok, I'm pretty much home. Should I quit while I'm ahead? Good idea to eliminate the relay despite my doubts. Autovalue had the relay for 24.00 dollars. I'm really not inclined to swap in another relay/harness/plug since it seems to be any deviation from the norm is usually the culprit in these headaches some way down the road. I went to the local junk perveyor and got one off of a '90 c1500. Got home, plugged it in, and it commenced buzzing. Oh well. It was a good idea. If anything, it was buzzing more consistantly than my original. Probably sign of a healthy unit! I removed the CTSY fuse again and got a good strong flashlight up under the dash. (one of a pair of anodized aluminum 9 led flashlights I just picked up at Harbor Frieght in Grand Rapids, MI. 2.99 for the pair with a clipped coupon. Holy crap these are bright!) So here's what I discovered. There is a terminal block on the firewall straight back of the parking brake. It has about 25 sockets with recessed spade plugs (never know whether to call these male or female. Even electrical systems are getting confused now'days) All were empty, except one. Probably an indication of the simplicity of this 'work truck'.   An orange wire was plugged into the top socket of the third row from the left. With a good light on it I could see that the socket had the letters GRN next to it. The sockets are idiot proofed (poka-yoked?) so that you cannot stuff the wrong plug into them. Interestingly, There was a slot on the first row marked ORG that accepted the plug as well. I switch these slots, and plugged the CTSY fuse back in, and presto!   Everything works as it did before. Brakes, turn signals, running lights (forgot to check the backup lights). All, unfortunately, except for the courtesy and cargo lamps themselves. Now the hell did all the outside lamps work for three months with this wire wrong? My courtesy and cargo lamps didn't work before, and they still don't. I was so sure that the CTSY lamp would work now that I fixed someones previous mistake, but they didn't. Oh well.   I can certainly live without the courtesy and cargo lamps. Thanks everyone here for your help and suggestions. And for putting up with my ill temper.

I wonder if anyone out there also has the HAYNEs Repair Manual for 88 thru 98 Chevrolet and GMC Pickups?On page 12-3 there are four photographs. The upper left photo, 6.4, shows the 'convenience center'. Upper right of this photo is a four pin inline plug. In my truck the left part of the plug is dangling and the right side is nowhere to be seen.Does anyone know what this plug does?   Right now my CTSY and cargo lamps are not working. Nor are my backup lights. On my 'convenience center' there is currently only one orange wire plugged in. No other receptacles are being used.   Does this seem right? If noone has this manual, I'll try to post a digital image tomorrow.