To expand on this good reply, it's likely you have water in the wiring. Virtually impossible to get out, and even if you do the wire wil already be oxidised and should rather be replaced.

Depending on the design of the radio, the backup current should be in the order of 10 to 50 uA, definately less than 1mA. The seat belt relay should be electrically disconnected when the ignition is off, and thus draw no current. But it might have a switched input and an unswitched output, so perhaps water at the input could cause a power drain on the output (if that makes sense to you).

On my 240 the fuel pump relay and driving lights relay was fitted hanging down, and got filled up with water. Amazing how long the car still works with water in there! Anyway, water can cause nasty problems and it is best to replace rather than repair. Have fun...