If talking about where the plate bolts to the unibody then probably tar based. Silicone will not mate up well with that. Soak part in solvent to remove the stuff or use what I often do, Home Depot roofing cement, which is very close to the same stuff. I use it all the time now on like windshields and gaps there and believe me silicone can't touch it and much cheaper as well. REAL messy though. Works well sealing underbody panel repairs too for leak prevention.