Tuskenraider,
Just got some info that the PRC8878 was for pre-1994 Disco 1's and this relay has 5 pins and it might not fit a post 1994 Disco. It depends on what relay holder you have and how many pin slots it has, as they (LandRover) changed the holders as and when they started including/removing rear quarterlight clusters. Typically, nothing electrical is ever straightforward.
The best bet is to check the relay holder on the fuseboard to see if it has four or five pin slots on it, before ordering a replacement relay.
The relay type you will need Tuskenraider is more likely to be a YWT 10003 Relay, with 3 pins. That's the relay currently in my 1998 Disco 300TDi although the holder has four pin slots on it, so I need to get the AMR 4873.
Good news is that the YWT 10003 is available on ebay for about 3 quid each, as opposed to 20-30 quid for the AMR4873.