Hi, I can confirm that one can be fitted "after" as I was able to grab one off a car that had been in an accident and written off here in Aus a couple of months ago. I took a chance on the wiring being there even though my original order did not have the compass ordered and when I removed the Grenadier symbol the wiring was there!
Removal of the Grenadier symbol that was there did take some doing and I did have to damage the circular body that held the Grenadier symbol by drilling out part of the holding pin that was held in place with a screw so to do it absolutely to OEM spec would require removal of the dash. I made the call to use a rubber grommet in the part of the dash that held the unit in place as the holding pin does "secure" the compass casing.
Having said all that, I connected the wiring as part of the cars harness, reseated the body of the compass in the hole (and it is not going anywhere i might add), and turned on the car, lights came up and the thing works.
As such the hard part is sourcing a compass and being prepared to accept that installing it will require a very small amount of unseen damage to behind the section of dash that held the bodies pin.