The ABS light came on after car was not used for a few weeks and a new battery fitted. The ABS light is now on all the time. The garage can't find the diagnostic plug.
Any ideas on either of the issues?
The first thing you should do is check for a blown fuse, it's very easy to blow a fuse when taking a battery out or installing a new battery.
There is an ABS sensor near each of your four wheels, if one has gone bad or is disconnected, that will cause your ABS light to stay on. It's a 1992, so the garage may have to forego using any diagnostic readers and test each ABS sensor, or worst case scenario replace all 4 if you want that ABS light to go away.
The ABS diagnostic port is under the hood, in the same assembly as the OBD port. I have never been able to get information from it, however. The best instructions I've been able to find did not work when I tried them.
I asked a question about this previously, but so far it does not have an answer: