Plastic doesn't make a good ground. Try metal. The entire frame for the dash is made of it, the steering column mount brakets etc...If you can't find a bolt that's already there just get a small self tapping one. You only need to ask on one thread btw, people are usually pretty quick to reply.
Edit: if you can see the fuse box under the dash you should be able to see metal somewhere around there. Or look for a bundle of black wires all connected at one place on a stud near the kick panel? Shouldn't be hard to find a place.