Found this on the How To section of the Online Imperial Club website:
Question from Jim (1968 side marker lights):
I'm going through the annual humbug of trying to get my '68 LeBaron through a safety inspection. I spent an hour today trying to get the bulb out of the side marker built into the front bumper. They're obviously set up differently than the rear marker lights, which simply plug in and out. I crawled underneath and spotted threads on the metal plug and finally managed with a pliers to get it to turn ... but then it just turned and turned and turned. So then I figured it had to pull it straight out somehow, but it wouldn't pull and I didn't want to break it. I couldn't spot any clips or anything that would suggest how it's secured in there.
Reply from Kate:
I have had these out on my Imperial, as the grounds were not working - they ground to the body through the casting of the light housing and the spring steel clip that holds them in. Take a stiff brush and clean the heck out of the backside around the point of entry through the body, then get a good light and take a look. The cast (zinc?) light housing had a flat metal slip-on clip, much like those holding rubber brake lines into those star brackets on the frame on GM cars and trucks. You should be able to tap the clip off and then the light comes out from the outside. It's a real neck-strainer to see up in there, though!
Hope this helps you, good luck.