Here’s a diagram for an easy way to add an auxiliary jack to your antique mono radio so you can play guitar through it. See my video below on YouTube too. The theory is quite simple — The volume control varies the level of an actual AC audio signal — Not audio high-voltage DC on a tube plate, or some weird radio IF frequency, so you know it’s an easy place to put an auxiliary signal in. The only downside is you don’t know how strong that signal is going to the volume control, so if you plug a guitar directly in there, you may have to crank up the volume all the way. There are possibly better places to put the guitar signal in that would give you more gain, but you’d have to trace the circuit, understand it, and then put your aux jack in — It’s a LOT more work.