What do I use to stick a "plymouth" script?

I wouldn't use any silicones. If not using tape, pick up some 3M Trim/Emblem adhesive.

[ame="http://www.amazon.com/3M-03601-Plastic-Emblem-Adhesive/dp/B000CCM3J2"]Amazon.com: 3M 03601 Plastic Emblem and Trim Adhesive: Automotive[/ame]