Use a 1p2t or 2p2t switch. 2p2t=2 pole 2 throw
So, the switch you plan to add, instead of ON-OFF, get a 2pt2. 2t is like an A/B switch. The circuit is connected to either A side or B side.
1p with you switch the V+ only. Use 2p if you want to switch both +V and Ground
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Rick