source to tap into interior dash lights - aux switches

[Lucasd2002]

I'm adding some auxiliary switches to my truck and the new switches have connections for the dash light circuit so they can illuminate at night or whenever the headlights are on.

I was planning to add a posi-tap onto a source wire behind the dash.

Is there a typical source that people use for this? Instruments? My vintage air HVAC? Does it matter where I tap in?

Thanks in advance

truck is a 1988 GMC crew cab v3500.