These light bulbs fit very well in your Peterbilt cab 3939 or 3935 and look amazing and are very bright! I am a hobbyist who founded the TycoUS1Trucking-com website that also has a full 10-minute video tutorial on changing the bulbs. Some brief clips of the video are in this listing. I will be happy to help you with any questions about bulb replacement which is fairly easy. Tyco US1 Electric Trucking #3935/#3939 Peterbilt Cab Light Bulb. TRUCK IS NOT INCLUDED! This is a replacement light bulb for your Truck. I also sell other US1 Trucking bulbs so check out my other listings! I have been a seller since 1999, so buy with confidence! Check out my other US1 Trucking auctions and I will combine shipping! https://www.ebay.com/usr/actormike Tyco US1 Electric Trucking Replacement Light Bulb Video will open in a new window Using the App? Paste link into a browser window: [isdntekvideo]