Your method of getting the DME engine temp sensor to work is interesting. Good work.
Your signature line should be corrected ---
1984 Porsche 944
The rest of your electrical problems need to be worked out with the wiring diagrams.
Pelican has them here:
Pelican Parts: Porsche 924/944 Electrical Diagrams
The tach should be OK but there are many on ebay to begin to get everything back to normal.
GL
John