Cadillac CTS Owner's Manual page 319

If coolant is needed, add the proper
DEX-COOL coolant mixture at the
coolant surge tank.
If no coolant is visible in the surge
tank, add coolant as follows:
1. Remove the coolant surge tank
pressure cap when the cooling
system, including the coolant
surge tank pressure cap and
upper radiator hose, is no longer
hot. Turn the pressure cap
slowly counterclockwise about
one-quarter turn and then stop.
If you hear a hiss, wait for that to
stop. A hiss means there is still
some pressure left.
2. Keep turning the pressure cap
slowly and remove it.
3. Fill the coolant surge tank with
the proper mixture, to slightly
above the FROID/FULL COLD
line on the side of the coolant
surge tank.
4. With the coolant surge tank
pressure cap off, start the
engine and let it run until you
can feel the upper radiator hose
getting hot. The upper radiator
hose is the top hose coming out
of the radiator, on the passenger
side of the vehicle. Watch out for
the engine cooling fans.
By this time, the coolant level
inside the coolant surge tank
may be lower. If the level is
lower, add more of the proper
mixture to the coolant surge tank
until the level reaches the
FROID/FULL COLD line on the
side of the coolant surge tank.
5. Replace the pressure cap. Be
sure the pressure cap is
hand-tight and fully seated.