You can go the Lestronic web site and download a manual for it which will include a wiring diagram.
Click the link below.
When the web page opens click on the
document search in the little menu in the upper left. When the search form appears
type in your model # ( 17120 or 39390 ).
You should be good to go !
http://www.lesterelectrical.com/products/work.htm
p.s. On my lester the manual says they use black for negative and red or white for positive. BUT, it is better to check for your specific model. Good luck.
You can devise a reminder for everyone. Or hang a sign on the steering wheel to let the children know when it is plugged in.....