I've tried aftermarket relays from several sources, while they will plug into the harness, they won't fit into that small box they have to fit into.
Here is a link to the OEM service manual for your drive, there is a wiring diagram in it incase you don't have it. http://www.4shared.com/file/48536730/fa3b3311/280_to_DP-C_Drive_manual.html
There isn't a test for the relays in that manual however, so here is that procedure from a different manual.