[Edit] Solved, see my reply below.

Hoping someone has an idea of what I could try next. This car is my daily driver and has been out of commission for about a month now. The problem is that fuse 13 immediately blows when I turn the key.

The fusebox cover says it's for the heated seats and turn indicators. This is either a bald-faced lie, or I have turned into a terrible, terrible shade tree mechanic.

The fuse has two circuits. Trial and error tells me the short is on the circuit at the front connector which I have determined has the turn indicators at least.

I have the following items disconnected:
1) Fuse 13 rear circuit
2) Both front Turn Indicators (wire insulation looks ok up there as well as inside the rubber covering)
3) Both rear Taillight assembles (which are new anyways, also the wiring looks ok there as well. I opened the rubber covering where it enters the trunk and all the wire insulation look great )
4) Bulb Failure Warning module removed
5) Door Chime module removed
6) Hazard Light switch (I thought this was the closest place to break the circuit)
7) Flasher relay removed
8) Instrument Panel removed
9) Both Heated Seat switches
10) front and rear Seatbelt warning bulbs

Even with all these items disconnected, the fuse blows immediately when I turn the key and energize the system. This is before the Starter engages.

I've traced the wiring from fuse 13 up to the wiring bundles under the dash. My next idea is to open up those bundles to continue tracing.

Does anyone know if there is anything else on fuse 13 or might affect fuse 13 once energy from the battery is flowing? If anyone has any advice I would appreciate it!

 

1987 is the worst year for the eco disintegrating wiring. This may explain the mysterious trouble you are having.

Have you seen any signs of wiring's insulation crumbling away? Inside that bundle you may have that going on and it'd cause all kinds of electrical gremlins.

 

I figured it out. This always happens when I struggle for a while and then go and make a post about it.

I went back to basics and traced backwards, starting with the Heated Seat switches. I had to remove the armrest and the housing over the parking brake lever and there it was. The Blue-Red wire that led to the passenger side heated seat switch connector was chewed up and exposed. It looks like it got caught between the teeth of the emergency brake sprocket and the lever. That makes the most sense as I had minor braking emergency a few months ago when my master cylinder failed and I had to use the emergency brake a bit to get home.

To review: 1987 240s have the Heated Seats and the Turn Indicators on the same circuit off of fuse 13. The heated seat wires can apparently get caught in the emergency brake lever apparatus.