Strange electrical issues w/ 86 Daytona Turbo Z

[Lee'sdaytona]

Greetings All,
Well, I am slowly making progress on my 86 Daytona Turbo Z C/S. Today I installed a radio for the first time-- a Mopar infinity, along with some speakers.

The radio works nicely and sounds good. When I open my door, and the dome light comes on, the radio goes out, as if someone pulled the fuse. The back lit buttons stay lit, as that is a different circuit. The time chronometer thing also blacks out. What gives? How can this happen when they are on two different fuses and circuits? How can an electrical load make another circuit stop working?

I don't know if it relates, but my horn doesn't work either, even after swapping horn relays. I tried to study the wiring diagram book, but I really don't know where to start looking and tracing. There were mice living in the car at one time. I think I got the nest removed, as there was a pile of insulation on top of the HVAC box, which I removed, cleaned, and replaced the heater core and A/C evaporator core. I have found no chewed wires whatsoever, thus far.
Also, my engine temp gage doesn't work. Is there a way to test the little one wire sensor on the cylinder head? And is it more common for the actual gage to go bad and not the sender?

Thanks for any help in advance!
-Lee

[Murphy]

I think you have a bad ground somewhere in the dash The temp gauge sender should have a certain resistance proportional to the temperature. Make sure there is a good connection to the sender and there is connectivity between the wire at the sender and at the gauge.

[Lee'sdaytona]

Ok, I did just replace the braided ground strap from the lower console where the stick shift cables go through the floor, because it was completely torn. The metal where it attaches was rusty, but not under the ground eyelet. I will clean off the rust and try that. Anyone know of other grounds under the dash to check?

How do I check resistance on the sender? There is one stud, so I assume while its running, but do I hold one on the stud, and one to ground? (cylinder head)?
Thanks guys!
-Lee

[Murphy]

there is also a ground strap on the back of the radio. Probably every device in the car has a ground wire going to it. I would really check out the one behind the radio, and going to the chronometer.

the temp sender you can measure at any time, even when the car is off. I actually recommend you do it with the engine off. yes you will attach one side to the stud and one side to ground. Hopefully someone can chime in with the resistance values. Make sure there is a good connection between the stud and wire. If you can connect your multimeter to the battery and stud using alligator clips so you can watch it without having to hold the leads in place and possibly have a bad connection there.

Not 100% sure on the horn circuit(dont have the diagram in front of me), but just start at the horn and trace it back. I would say its most likely the clock spring.

How comfortable are you with working with electronics and measuring voltages and resistances, tracing wires, testing relays?

[Lee'sdaytona]

I'm pretty good with a multi meter, and can hold my own understanding resistance, voltage, relays, etc. The difficult part for me becomes tracing wires that are hidden under dashboards, and in conduit. Is the clock spring an actual part of the horn, or is that the electrical contact in the steering wheel?
Thanks,
Lee

[Murphy]

in the steering wheel