A single DIN aftermarket radio will fit that opening. The lower opening was used for a storage pocket. The wiring is easy when using a wiring diagram. You can use the radio console from a later 960 (1994+) to mount the OE Double DIN radio.

ABS tone rings on the front hubs may need cleaning to remove rust. You need to remove the front rotors to check the tone rings. Temporary test would be to unplug the front ABS sensors, which are found on the inner fenders under the hood (bonnet), to disable ABS then test drive to check if the car st...

Looks like you are going to be removing many AC parts. Not sure if the circled part is a radiator or an intercooler. Probably locate some used parts from a junkyard car so you can take it apart first so that you know how it comes apart. Then putting your car back together won't be such a mystery.

My guess is making sure there is 12 volts and a chassis ground reaching the seat controls.

Loosen head bolts in reverse order pattern (starting at the ends).

Radio Suppression Relay poor wiring connection will cause no power to the injectors. You'll find this on the inner fender.