Could installing an aftermarket radio on a 1996 Subaru Legacy be the cause of the dash lights interm wipers dimmer switch etc to stop working The fuses all check OK?

Usally not. The only link I see is that these are usually on the same fuse. Ohm check the fuse. Also check that the dimmer switch just isn't disconnected or turned off\down.

-edit-

Possibly - I installed an aftermarket stereo on my Subaru Loyale (1990) and had the same symptoms you described. After troubleshooting for months, I discovered that during the stereo installation I fried the actual dimmer switch. When I replaced that, everything came back online. If the fuses check out, try replacing the actual dimmer switch. You can check this by bypassing the dimmer switch - the dash lights should come on full-power if this is the cause (connect the wire going into the switch to the wire going out of the switch).