In the process of doing a heater core replacement on my 1990 AWD Legacy L. It took me about two hours to get the old core out. I found it was easier to simply take the whole console off--including taking out the radio/heat/gauges separately. Also easier if you disconnect the AC box from the firewall.
I thought this write up was rather useful:
http://www.ultimatesubaru.org/forum/showthread.php?t=27537&highlight=leaking+heater+core
Still waiting on my core to show up. The car will run and drive without the dash, you just lose your turn signals and high(?) beams--brake lights work.
