"Does anybody know roughly how much current one could safely put through a 20 amp (125V) toggle switch....at 12V? Thanks"
It's not what you can put through it but what you can break . The contacts will probably carry even more than 20A, but I would take that figure as the maximum unless you know otherwise. Even when they claim 20A at 125v that is likely to be a resistive load rating ( heating/lighting). Reactive loads such as motors will generally have a lower rating. Some switches have 2 ratings marked on them and the high one is resistive and the low one is motor and inductive load duty.
If it is any reasonable sort of switch with even a hint of "quick break" then it should manage 20A resistive at 12v. The inductive rating will be much lower and unless it is a true " quick break" it will not be happy on dc inductive load unless the load is bypassed by a freewheel diode. For a modestly inductive load such as a small dc motor I would limit it to about 5A. For highly inductive loads such as starter solenoids then it may not manage 3A long term reliably without diodes or contact suppression.
Switching inverters and electronic things may also impose rather a severe limitation with the capacitor charging current and such current surges are not normally covered in the rating.
For lights and heaters treat it as 20A but for any other load expect a large de rating.
Flux
