For the minimal current requirements of an oil pressure gauge, I just
tapped into the radio power lead.  Make sure the wire you use has 0 volts
with the key off, 12V with key on.  I pulled the factory radio long ago,
but if I recall correctly there are two power wires, one always hot, and
one switched 12V.

As far as the gauge light, I spliced into the climate control back light
which is dimmed with your dash lights.  It's very accessible if you just
pop the climate control panel off.

When splicing, I prefer to cut the wire to be "tapped" into and
solder/shrink tube the tap wire in.  No gremlins after this way.