Unfortunately I failed to measure the lamp and the replacement was a size bigger festoon. Not to be one to give up, I had a look on the Interweb and you could only seem to buy adaptors with LED panels, they come with three types.
The Instructable is about how I made the adaptor rather than the device it powers which is made out of an old magnetic catch body, two crimp eyelets and the spring out of a scrap 12V Plug. I cut the lugs off and shaped it to fit inside the light fitting but it was still a tight fit.
Materials: Festoon Lamp. Solder. emery paper. 8mm plastic wall plug. Light guage flexible wire.
Step 1: Measure the overall length of the festoon bulb to be repaced by the adaptor.
Step 2: Remove caps from old festoon bulb. One fell off the other needed some heat from my torch and a lot of brute force and ignorance. Clean the caps inside and out using Xacto knife, drill bits emery cloth etc.
Strip about 5mm (3/16) insulation from the wires. Make sure the hole in the end of the caps is clear with Xacto knife then poke the wires through, tin, bend over and solder. That is the wires you bend over, if your bench is the right height you shouldn't need to.
Step 3: draw two lines on a piece of card or something to represent the finished length you are aiming for. Position the two caps with the points over the lines. Offer up the wall plug, you are going to cut it so it will space the caps apart to this length. It does not need to be that accurate as the spring in the light fitting is very accomodating and you can bend it out if needed.
there is already a slot in the plug to accommodate one wire and you can cut a sliver out of the other end wide enough for the wire and about 1/4 long, offset from the existing one so they don't meet end on. The plastic on my plug was very soft so you have to be careful not to overcut.
Step 4:Pop the caps on the plug and the adaptor is ready to connect to whatever you want (within reason) but probably an upgraded interior light of some sort
You could glue the caps to the plug but I didn't as it is a snug fit and the spring in the fitting keeps everything in place anyway
Sorry only one photo but I wasn't planning this as an instructable.