Check this from the 2005 Rules (PDF format)
A. Accessories, gauges, indicators, lights and other appearance,
comfort and convenience modifications which have no effect on
performance and/or handling and do not materially reduce the
weight of the car are permitted. This does not allow driver’s seat
substitutions, or the removal of “tow hooks”, a.k.a. “tie-down
loops”. Delayed shutdown devices such as the “Turbo Timer”,
which perform no function while the car is in motion, are permitted.
This does permit the installation of an additional mirror (e.g. a
“Wink”), but does not allow the removal of the original mirror.
“Grounding kits” specifically designed to support sound systems
are permitted but may serve no other purpose.