I modified my Mesa Mark-V 35 head so that I could connect a TRS (stereo) 1/4" phone plug directly to the back of the cabinet to control the two most critical functions provided by the large footswitch included with the amplifier. First, I never liked the huge pedal and it always made a resounding "clank" every time that I stepped on one of the footswitches. Also, I had the special cable go bad and I had to order another one.
Here is what the Mark-V 35 looks like out of the cabinet. The area circled in RED is free of obstructions and can be used to mount another jack.
View attachment 3390
I chose to use a simple 1/4" stereo jack to wire in parallel to two of the pins on the DIN connector shown just to the right of the red circle.
View attachment 3391
I can't remember the particular pins that I used on the DIN cable, but a description of which pins service which switch functions are described in your owner's manuals or downloadable from the interwebs. I just selected two pins, one for channel-selection and one for the solo-boost. I never switch off the reverb or the EQ, but rather assign these features once and leave them alone. This allows me to connect a more "convenient" footswitch that has the property of near-silent switching, and provides better convenience to hookup external switchers. Most programmable switchers can address both of the switching circuits on a TRS cable anyway. I just made this out of an old Morley A/B switch:
View attachment 3392