I’ve just started a new job today, and im drivin a 57 reg volvo fh12. Its a lovely motor, havin only driven dafs mans and mercs it feels like luxury 
. The only problem i have got is i cant get the 24v socket to work. i have tried the fuse but its fine and the connections on the socket itself, but i was wonderin if anyone has had the same problem and knows a quick fix. The thought of spendin the next 3 nights just readin a book doesn’t seem to apealin.
There’s a good chance the 24v feed to the acccesory socket has been ‘borrowed’ by a fitter to supply something else like a CB or DVD player etc.
Take the fuse out, and see if anything obvious stops working.