Head unit not working after changing battery

[Mr. Nice Guy]

My head unit has stopped working since I changed the battery in my MG a few weeks ago, it's a classic car with a modern unit in it and it appears to have been wired by a chimp, so I can't just turn to google for help. As far as I can see there is no power going to it at all. I have tried reconnecting the battery a few times, both positive first and then negative second and the other way around. I realise now that there is an order you are meant to follow when disconnecting but who knows which way round I disconnected and reconnected it when I first changed the battery. Could following the wrong order have fried something? I have also checked the fuse in the back of the unit and it appears to be intact. Wiring is not my strong suit and it's very fiddly to get to any wires under the dashboard, so I'm trying to avoid tracing cables around. Any ideas?

[TooMany2cvs]

Mr. Nice Guy said:

My head unit has stopped working since I changed the battery in my MG a few weeks ago, it's a classic car with a modern unit in it and it appears to have been wired by a chimp, so I can't just turn to google for help. As far as I can see there is no power going to it at all. I have tried reconnecting the battery a few times, both positive first and then negative second and the other way around. I realise now that there is an order you are meant to follow when disconnecting but who knows which way round I disconnected and reconnected it when I first changed the battery. Could following the wrong order have fried something? I have also checked the fuse in the back of the unit and it appears to be intact. Wiring is not my strong suit and it's very fiddly to get to any wires under the dashboard, so I'm trying to avoid tracing cables around. Any ideas?

Do you have a multimeter? If not, get one. A tenner. Without one, you're guessing.

[marshalla]

Anything else stopped working ? Any other fuses you could check ?

[defblade]

The order for dis/connecting the battery is much more about your safety than bothering the electrics (take negative off first, reconnect negative last - with the negative connected, effectively every metal part of the car is also the negative pole, making it much easier to short circuit the battery if you're working on the positive connector. Hence why there's usually a red cover over the positive, but nothing on the negative).

As said, you need a multimeter, or at the very least a circuit testing screwdriver. Or even more least, a mate with either of those

Running an old MG, a multimeter will be your friend forever!