My roommate's 1978 Volvo 245 (yep we both have basically the same car, mine's got the better engine) has an electrical problem that I hope will be simple to fix. The radio and the fan won't work at the same time, if you turn the fan on while the radio is on they both stop working, accompanied by a "clonk-a-donk" noise from the left side of the dashboard. The radio will only turn on again after leaving the car off for a couple hours. I noticed that none of the fuses in the box control the radio or the cooling fan, do these two things have a circuit breaker instead? Is there a relay that's getting stuck?

 

sounds like a relay issue. I would pull off as much of the panels as possible then simulate the problem, see where that noise is coming from, then replace that.