Rather than just tell you, I'm gonna show you how to figure this out.
I've never had a '69 Dodge tail light out, so I know nothing. Well, really I'd have an educated guess, but follow along with me... Here's how I'm going to figure it out if it was my car.
First, open Google and type "1969 Monaco tail lights" and hit "search". Now just search for images. I just did this and came up with some pictures on eBay (which is always a good place to get pictures). Going through the pics I see some that show the tail light, but I'd see any screw holes on the outside. Go through some more, and voila, there's a pic of the rear of the tail light housing. I see studs sticking out and then phillips screws holding the lens to the housing. So, the studs have to go in holes somewhere and they are right next to the holes for the light bulbs. I know that a lot of older Mopars let you change light bulbs from inside the trunk, so chances are the studs poke into the trunk.
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Now, the easy way is to just open the trunk, but let's say I want to verify this. Google "69 Monaco trunk" and not surprisingly, I end up on the forum with some car that was for sale and there are some pics that show me I'm right that they are removed through the trunk. Quite frankly, I got a little lucky finding this one, but the pic shows the tail light wiring and the nuts for the studs.
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It would have been easier to just tell you "it's in the trunk", but I go back to when I worked for a living and had to teach new guys how to do things. I always figure it's the "teach a man to fish" that wins out for everyone in the long run.