Ok, finally sometime this evening to get working. I installed the +5V to the P1900 and set the jumpers. Hopefully I set the jumpers correctly. Im using a MACPAC cable so powering the MacMini was a snap. I'll use the secondary power for the Xenarc monitor. I'll use the new third power supply for the USB or AMP, not sure yet.
Large book in the back is the Porsche 944 Electrical manual which shows 3 options for Radio installiation and Im not sure which one I have. One shows a seperate AMP installed somewhere in the car already which would be ideal. Id like to tap into what already exists since I have not purchased the amp yet. (Not likely my car though.) Checking with the Porsche 944 guys on this.