Inline car radio AM modulator

I built a xtal controlled am transmitter for testing and repairing valve sets, it works well.
I'd like to do something for working on and displaying am car sets, and to keep legal not actually transmit but put the signal source inline with the sets aerial.
I was thinking of a ferrite core with windings for aerial and modulator input, not totally sure how I'd go about this with a high impedance aerial input like most of the older sets have.

This may help:
http://www.antiqueradios.com/forums/viewtopic.php?t=72449&view=next
There are links to a zip file with the schematic etc and to a Youtube video, at the above site.

Here's another site, which discusses the problem theoretically:

http://www.discovercircuits.com/dc-mag/Issue_12/pg-4.htm

And lastly, this:
http://www.flashwebhost.com/circuit/miniature_mw_transmitter.php