There are things such as signal boosters. They are popular for TV and Wifi, but they are also available for FM. The higher the DB gain, the stronger the signal increase. Some FM signal boosters claim, 15db, 25db and even 36db. There are some Wifi boosters that claim 300db gain. I would try a FM signal booster. Some people claim they work, some people will say they don't work that well. A lot of time, you have to change the setup on them. Sometimes not connecting the ground or even reversing the alignment might make it work better. Although, some of them might claim higher DB ratings, your experience with a particular one, might be different. You might want to find one with a lot of good reviews.
I actually remember seeing a thread where someone had the same problem after wheeling and solved the problem. I think I remember that something got wet and the ground wasn't as good around the antenna. If the antenna is being grounded to the chassis, maybe you want to remove the paint wherever the antenna is grounding. I would also check to make sure your radio is properly grounded. Just because your radio is connected to a ground wire, doesn't mean the ground wire is actually connected well to the chassis. Sometimes when the stock radio is removed, they disconnect the ground at the chassis. So, although the ground wire is still there, it isn't actually grounded to the chassis. This was true for me. I traced the ground wire on my radio, and I found that it wasn't actually grounded to the chassis on the other end. Maybe even try grounding your radio to a better ground source.
This link might be informative, or look for a radio signal booster.
How to Improve Car Radio Reception | eHow