Had a similar problem with this (I think) circuit on my 83 300d. I remember that it killed the horn. Thanks to this forum I learned that the kick down on the transmission can short to the case and kill this circuit. You did however say it blows immediately. As soon as the car is turned on or after a little driving? In my case, it blew as soon as I hit the kick down switch because the single wire running from the throttle down the drivers side of the transmission to a solenoid on the rear can get it’s insulation rubbed off and ground to the transmission case (or other metal it touches). A little electrical tape fixed it for me.

__________________
'83 300D turbo diesel (334k daily commuter)
SOLD '80 240D 4 speed manual, #2 piston pitted from crushing a 10mm nut. Running it 'till it burns through.
'78 F-150 300cid 4spd od (with my great unkle's original sales receipt)
SOLD '66 Ford P-350 delivery truck (almost driveable)
'49 Dodge B1D (1 ton Pilothouse era truck) not running but all there, candidate for om617 and 4x4 conversion when money permits.