Try turning on the headlights, the air conditioning, the heated rear window, the heated seats, everything that eats electricity in your car. If that doesn't cause the Mark2a to "brown out", then it is likely that your alternator/battery/regulator are OK. If under those circumstances the player will works properly with low beams on, and the high beams make it brown out, then I'd start looking for a problem with the high beams, some sort of unusual current draw.

tanstaafl.