88: 12V constant (blue/yellow) is very low at 0.400mV
 

I picked up a cheap bluetooth stereo from Fry's a few months ago and went to stick it in my vehicle. I found a wiring chart online somewhere that said I'd be looking at the grey wire for ACC+ 12V and blue/yellow for constant 12V. In ACC+ I do get 12V from the grey, but my blue/yellow goes from ~400mV when off/acc/on to at most about 4V when running.

I hooked it up anyway just to see, and of course the guy didn't come on at all. I connected the ACC+ 12V to both my stereo's 12V switch and constant and the stereo came on just fine with the car, but that's not a long term solution.

I'm guessing it's an electrical problem, probably a short, but I have no idea where to look for it. I examined the wire as well as I could back through the dash and it looks okay, but there are no places to probe. I don't have any other electrical issues. My dome light seems bright when the car is off and I'm getting 12V off it.

I never paid much attention to the old radio, which didn't have a clock, so I don't know if it's existing or if I caused the problem by disturbing the wires.

I'm confused by the change in voltage from 400mV to 4V when I start the car. I feel like that could be explained better by a faulty wire than a short. That means I'd need to run a new wire, and I'm game to do that, but have no idea the path of the wire.

If there were another 12V constant in the area, I could also just steal from that, but I don't want to fry the wires. I assume the clock has something, for example.

Anyone have any ideas?

You mean Blue/Yellow?

In fact I did. I was looking at the wiring diagram on my phone when I was probing wires, but then went from memory when writing this. It also lined up with the yellow wire on the harness from my old stereo.

Maybe I should drive a bit and then probe again to see if there's any change as things shift around.

Does anyone know if the constant 12V for the stereo is on the same circuit as anything else? I can't imagine it's as simple as a direct attachment to the battery.

Constant power sources that may be in the area.

Dome fuse powers (if the car has these), map light, ign key light, door light.

Wire colors are blue/yel to junction, then red/green. For the map light color changes to red.

Power to stop light switch under dash at top of peddle (green/red wire). Stop light fuse is 20A.

Hazard switch power (white on US cars-green on Canada).

Was a clock optional if so is there a harness plug for this having another blue/yellow wire?