I have sorted this out now thanks to this wiring loom harness:
http://www.ebay.co.uk/itm/140783085245

One of the connectors allowed me to connect the radio/stereo to positive (+), ignition and negative/earth. The other connector is meant for the speakers but I didn't need that since I was lucky to find speaker wires on eBay with the original plug that fits into the amp. I already had the wiring between the radio and amp in place so when all was connected the radio and cassette player now works like a charm.

Regarding my third question I did found an amp with front panel that fitted the 2wd dash. Except for the front panel the amps are exactly the same and you can dismantle the panel if you want to switch. I did switch the panel to an amp that originally had the 4x4 dash panel since that was new and never used before.