Painstakingly wired up a 4-way switch in the old Squire tele I do my experimenting on. It works, and sounds great and all, right until I apply any gain, then it squeals and farts, feeds back, whole guitar is microphonic to the extent that I can hear the strap squeaking and the buttons on my jumper sliding against the guitar body.

Checked all the grounds throughout...

Lined every cavity with aluminium tape...

Resoldered every joint...

Still NFG.

A bit of googling and I came across this tele base plate issue, wherein the good Japanese Fender pickup you've had in the shed for ten years starts to come apart, and the wax that holds the base plate on dries out - so I took the strings back off for about the third time tonight and pulled the pickup. Sure enough as soon as I interrogated the copper base plate with my screwdriver, it fell clean off.

Long story short, I'll report back tomorrow if a bit of NEC tape has cured it... Pickup is currently relaxing in the vise, and I'm having a beer!