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?