Hope someone can help give a few pointers on what I hope is a simple problem, but as I'm not that technically minded I'm at a loss.

A friend has a Polo with a cheap Goodmans unit, it managed to blow a fuse, not the one in the main fuse box, but rather one of the "in line" ones that's in the little black box in the wiring loom which goes straight into the back of the unit (hope you know what I mean).
In this little black box is something, dunno what, capacitor? transistor? who knows, along with two fuses, a 7 amp and a 1 amp. It was the 1 amp that went for no apparent reason, I have replaced it, however the new one blew after a day, so now I need some help in getting to the bottom of the problem. I'm hoping it has something to do with the head unit as she was looking to replace it anyway, however I'm conscious it may be an underlying problem with the car's electrics somewhere.

It may be unrelated but she's also had problems with the battery, two new ones in 2 years, the most recent being c. 9 months old, the car often has trouble starting and when sat for long periods eg 4 weeks or so it needs jump starting.
I'm not that familiar with car electrics, I have a multi meter but to be honest I'm not that proficient in using it, so any pointers/help on the above would be greatly appreciated.

Apologies if I should have posted this in Other electrical, feel free to move.

Thanks in advance.

Steve