[MrFixr55]

This may be ugly, but cutting down a radio chassis from about 8" to about 4" is not pretty either. Are you merely using the front of the radio as a filler for the hole in the dash and a support for the control or did you actually preserve the radio RF / IF / Detector section of the radio? Are you opening the Bluetooth amp and running wires from the volume control for the amp to whatever control you are using? Do you need only volume or are you expecting the tone control to function also?

I assume that the BT Amp is Stereo but is controlling both channels with some sort of voltage control input, and this is why you need a 1K linear pot. If you need a pot element for each channel, then the tone control also has to be converted.

Judging from the pic, you are almost there. I see little difference between the element of the tone control that you disassembled and the new pot that you disassembled other than the shaft hole in the element. Are the outside dimensions the same or is the new wafer smaller? Difficulty is the stacked pots. If you can open the volume (there may be enough room to bend the tangs back) and retract the shaft, you should be able to remove the 500K Audio pot element and add the 1K element. An alternative is to use a 1/4" coupling (available from a good hardware store or (gulp) Home Depot), cut down the shaft, glue them together, mount the new pot and use the coupling to join the shaft. The volume and tone controls will be permanently joined together and rotate together. This is a last resort, so make up a dummy shaft first to make sure that this will work. A professional custom pot maker may need the original shaft unmolested if this would not work.

For any alternative, pix of the radio (or its remains) are needed. Pix of the BT Amp will be useful also.