the led should not be lit. that could indicate a problem.
http://arcarc.xmission.com/PDF_Arca..._Midways_MCR_II_&_III_Systems_(July_1983).pdf
that faq, i believe either page 7 or 17, can't remember, will tell you where to check and adjust.
http://users.rcn.com/jenison/mars/faq/MCR_FAQ.txt
the mcr faq will tell you how to use the dip switches to see if there's a ram/rom problem.
the first thing i'd probably do though is check the cables. they're typically real brittle and can cause issues. the thing that i think is more important than reseating them is to make sure there aren't any bends/kinks that can cause lack of signal. what i do is pull the boardset and test each end of the pins starting at the top of the cable going down for continuity/resistance with a multimeter. if all 5 cables aren't working, the main things to have hooked up are the video board (that sits against the cab side) and the cpu board (the middle one). if you can get the 3 cables for those to be ok, it should give you an unplayable game with an upside down screen. that would at least indicate the boards are working even if all the cables are not.
if the cables are bad, bob roberts has them for $15 / set plus shipping.