Twenty years ago when I was last fitting aftermarket stereos into cars there were two +ve connections; a permanent live for things like the clock and a switched live from the ignition to turn it on and off.
For future reference, if it’s a relatively modern car (10 years or so) you can get a kit for £20 or so that plugs into the original radio connector and decodes the canbus signals so it knows when the car’s off or on, when the headlights are on and for satnav units how fast it’s going etc. I had one for the Passat that had the VW connector on one side and the standard radio connector on the other.
when I installed one in an old Bora, i had to swap round the yellow and red cables, i seem to remember they were attached to plugs, so it was very easy.
So, red to yellow, yellow to red……I phoned Halfords about this at the time and the very nice (obviously pissed off charging people for nothing) radio specialist said they were charging people 35 quid for a radio fitment and this was all that was causing the issue