Adding this for historical purposes. Found many threads about this topic, but it would include using expensive hardware or swapping of molex connectors with some other work around. I just wanted one switch on the front on my cab with out spending $150+. Text is copied from my project thread.
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When I originally wired my cab I wasn't thinking about jamma (I knew I wanted a jrok board but I also wanted to play games in the meantime) and to be quite honest I really didn't know anything about jamma and figured I would crash course it when the time came. Upon investigating the hardware required for mame pc to jamma board switching options things were getting pricey. I was reading a review on klov about a jamma switcher and someone chimed in "why bother, do what OPs do and get a handful of relays and wire a switch to the 12v and have the relays switch video, audio etc...". I'm really surprised I didn't stumble upon someone else s mame/jamma relay project with the various search wordings I tried on byoac and google. Any-who, Took a quick trip to radio shack and snagged 7 relays, 2 1\8" TRS female audio jacks, 2 female 15 pin vga plugs and a low profile rocker switch. After some thinking and a good long soldering session I now have on the fly mame pc to jamma board switching for under 55 bucks OP style. When power is applied to the 7 relays from rocker switch on front of machine they handle the switching of video, sound, jamma +/-5 board power, toggle of ipac power (jamma mode) and lifting of ground from jamma board (in mame mode). I used the ipac as a splitter patch bay of sorts to get controls needed to the jamma loom without having to muck up my current mame wiring.

Overview of wiring.
Hope this helps someone in the future that wants to add a jamma set up to an already wired mame cab that only has an ipac or similar.
good day.