Weird I mean WEIRD electrical problem

[bitofaclue]

So I've just finished my long block swap into the 97 GC 4.0 auto.  It's up and running, if a little roughly.

History:  Last November, after nearly 500k km, the engine blew.  Then it sat until last week when I tackled the swap.  So it was without battery for about six months.  Also, before the motor went I'd had no electrical problems with this thing, weird or not.

Now:  A bunch of seemingly-unrelated things don't work.  The hazard flashers, turn signals, radio, dashboard computer, interior lights, and power seats are dead.  Perhaps the oddest of these is the hazard lights, as they should work even when the key is out.  Everything else works.  But I made a strange discovery.

If I hold the headlight dimmer switch in (that is, pulled toward me) when the high beams are on, all of the dead things work.  However, toggling the dimmer and holding the switch (again, pulled toward me in the low-beam position) makes all those things fail.

I've checked every fuse and swapped every relay possible.

Weird.  Consistent, but weird.

Any hints?  My first thought was the multifunction switch itself, but so many of these things (the radio and power seats, for dog's sake) have nothing to do with it.  So is it the Body Control Module?  I'm an EE, and this one's got me stumped.

Thanks.

Jonathan

p.s. The reason I mention the time it's been stored is that I'm wondering whether there's some kind of battery backup in the BCM that might have gone dead and caused it to lose rational settings, just like a PC  losing its BIOS settings when the motherboard battery goes flat.

[vantagetes]

Check your ground.

[Greg-100]

Grounds are a good place to start.

Also find a wiring diagram for that vehicle, see where that circuit gets it power from that feeds all the things you mentioned.

The problem might be no power is provided at that junction, hence no power to a number of unrelated things.

When we where kids, my brother figured out on a 66 ford custom, if you turn on the fourways, step on the brake with the signal light on, you could listen to the radio without the key.

Greg

[bitofaclue]

Yeah, a ground problem seems like the obvious answer, but...

When I pulled the motor I left the harness in place in the compartment.  As far as I can tell, apart from the alternator and starter there are only three ground lugs that get bolted to the motor - all on the passenger side.  One's just above the starter, and the other two are on the bracket that holds the coil.  I've been all through the engine compartment and can't see anything amiss.

[bitofaclue]

Problem solved.  Though I have strong resistance to believing in coincidental failures, that's what happened here.

Not a ground problem at all.  One of the fused but unswitched +12 wires running from the Power Distribution box (next to the battery) to the Junction Block (down in the passenger footwell) had a nick in its insulation where it passes through the firewall.  The nick let in moisture, which eventually (concluding sometime in the last six months) corroded the wire in two.

If I hadn't spent the $100 on the factory manual for this thing when I bought it seven years ago, I wouldn't have stood a hope of finding the fault.