Heads up that the 85 is an entirely different animal than the other 80’s turbos.
The tach amp is behind the 85 glove box with a 10a blade fuse on top.
You can probably use the 83 turbos amp and the 240 pickup but you have to do your own wiring for 12v, god, signal low, & amplified signal. Sounds like you already know this.
Images of amp in post 31 of my 85 thread.
Got a 1985 300D turbo
Edit - ugh like usual I spoke too soon. The little silver box under the fuse may not be the tach amp. The fuse supplies power to the tach amp electronics which is likely in the EGR controller that you do not have. I’ve been searching around and I cannot find the thread about redesigning the tach amp. They had pinouts and schematics for the 83 version which can help you with your rewire job.
Here is the schematic. A little fuzzy but it has all the pinout numbers for the wiring.
How the Tach Amp works