Hey guys, okay first of all, i don't own a mesa or orange amp, i just own a Bugera(can't afford the high end amp) but the Bugera it self is using a 5 pin din for the footswitch jack, i recently just did my own 5 pin to a stereo jack for the relay on the fx8. But i just can get it to change just two channels though, still have to figure it out for it to change to one more channel.
Here's what i did, first you need to check the footswitch of your amp and see what cables and resistor are they using for the amp switching channel. As for my amp the clean channel need a 100k resistor, crunch 1k resistor, lead 3.3k resistor. And you need to find out which cable are the power to power up the channel switching(which usually are white for most connection that i know though) and im not an expert to be honest, so u will have to find the diagram first.
2nd, you need to find out which cable are used to switch channel to 1st,2nd or 3rd channel. Then just use that cable and solder it to tip or ring to a 1/4" stereo jack.
3rd, assuming the power cable are white, you need to split them (depends on how many channel your amp have) and solder a resistor(check them first with a mulimeter on the footswitch) and just solder it to each ring and tip. And done.
Hopefully this helps.