I hate electrical but;
If all was well until the 105, look hard at that rear plug, maybe go to the next
cannon plug up the line and disconnect it. and try the fronts.
Look at the bulbs, did something short and burn all bulbs?
Find the Circuit breaker that covers that, maybe do a continuity test
to see if it is open or closed as normal.
If the flasher and turn switch you swapped in were used they could be bad.
I think Eriks has those parts, it is a safety item so if you did
swap in used parts get them new. Consider buying a new CB too.
I know throwing parts at a problem is not the professional way to fix electrical
but the flip side is to hire a spendy automotive electrician...
Look at your GROUNDS too. ALL these trucks need to have ALL GROUNDS
disassembled and paint and rust removed to bear metal, use combo internal/external
star washers when you reassemble. I have a handheld glass bead blaster that
does this job quick and EASY. I used dielectric grease on all surfaces during
reassembly. Make all TIGHT after cleaning.
I added a ground from the Left, Rear, Bottom of the Cab TO the frame
I added a ground from the Bed TO the Frame when I pulled the fuel tank ground.
I did those w/ I think it was a #8 wire.
Some of the grounds;
Alternator to Frame
Frame to Compressor
Hood to Frame
Cab to Frame above starter
Starter to Engine
Engine to Frame
Fuel Tank to Frame
All Battery Posts need to be cleaned to perfect and fix all battery post rubbers AND
THESE TRUCKS HAVE A NASTY HABIT OF THE BATTERY MAIN CABLES WEARING
ON THE LEFT, REAR of the engine block. It is impossible to see. The only way
I could do it was to disconnect them at the STARTER and move them around.
I found one of these REALLY BAD PLACES WHERE ALL INSULATION WEARS AWAY.
I slipped a very thick piece of rubber hose the right size over the cable.
In the future "I" will move batteries..
Find ALL Grounds and pull them ALL and clean ALL and reassemble.
All these trucks need this PM IMO.
The two bolts that hold the turn/stop lights to the truck ARE THE GROUNDS, pull and clean.
be ready for those bolts to break and you will need new lights.
Follow all wires, look/feel things look for breeches.