I would pull the lens off and look at the socket. You indicated that it illuminates, but does not flash, providing it is the correct bulb, if memory serves it is an 1157, assure that the larger of the two internal elements is illuminating and not the smaller of the two. If it is the smaller, get a short jumper wire and touch the metal frame of the socket and run it to a known good ground and see if this makes a difference. While you're at it, pull the bulb and make sure the contacts are not corroded off, sometimes wiggling the bulb in the socket will reveal a poor connection. Give it a try and see if it gets you anywhere.
