Why is my stereo fuse blowing?

Hi all. I'm the new proud owner of a T4 which I am doing a few things to.

I decided to replace the stereo as it was the old original one (1999) and I wanted one with bluetooth handsfree and music streaming.

Anyway, the old one came out easy enough, just a firm pull did it so I disconnected it and connected up my new stereo. It wouldnt turn on, so I checked the fuse, all ok, so plugged back in old stereo to check, now that wouldn't turn on.

Everytime I check the plug for power, the 10A fuse blows. Plugging the stereo in doesn't blow it although it doesnt power up, only when I test the plug for power. Negative on a grounded bolt and positive on the power pin in the plug, and pop goes the fuse !

Also the previous stereo would sometimes drop the volume lower when I went over a bump. I would turn it back up and next big bump, down goes the volume again. I didnt notice if it was cutting off power and back on again.

Has anyone got any ideas please? I'm certainly no electrician so in basic terms would be nice :) What I don't understand is why the fuse blows when I test the stereo plug for 12 volts.