How to upgrade older MTH silhouette passenger cars

Truthfully, you'd be better off just buying the ones with interiors.  Building them up is going to be a lot of work.  If you have a somewhat unique road name, you "might" consider buying a similar set with interiors and swapping shells.  If you shop around, MTH cars can be had fairly inexpensively at times.  Since you wouldn't be looking for a specific roadname, that might be the cheapest and easiest way to do the deed.

The bad part is, the lighting is one central bulb on the bottom of the cars with the silhouettes 

sent you a email. just following Johns suggestion

These are Empire State express cars that match a engine I have.  They are a nice aluminum car. I planed to add led lighting using GRJ’s modules.

any help appreciated 

Here's an easy alternative....replace silhouettes with clear plastic sheet.    Spray inside of windows with Dullcote.  This will block view into interior of car without the cartoon-like silhouettes.   Also works great for buildings without interiors, obviously.    John A

Rich, since these are MTH cars, try them first.  Do they have aluminum cars that use the same extrusion as your cars?  If so, you can probably buy interiors that will work in your cars with internal modifications to adopt them.

My reason for suggesting you contact MTH is that LIONEL's aluminum cars, at least the ones I'm interested in, use the same extrusion which permits use of the interiors introduced in newer production.  Most, however, require use of a different frame, and even trucks--so it's not without expense.

I've asked Midge a while ago...  interiors were not available from MTH!

Check out this link to LIONEL's website--lots of "interiors" (the search term) available, including K-Line, aluminum, heavyweights, etc.  Not every one, of course.

If MTH's extrusion was the same or highly similar to LIONEL's, this may be a good alternative.