540i Intermittently Stops - Alternator or Ignition Switch?

Car: 2002 540i Problem: Since last Jan., the car intermittently stops and based on feedback I've gotten here, I've driven the car for 5 months to test the theories (e.g., visor test, turn radio and seat warmers on while driving) and they do appear to impact the car. Everytime, I drive the car without turning them on, it drives FINE. But the minute I pull visor down or put seat warmer on the car idles abnormally and cuts off. Based on research on the forum, it could be 1) ignition switch or 2)alternator. My husband says ignition switch was replaced a couple of years ago but not sure it was done right. Don't trust shop used. Any ideas where to start? Dealership said they don't know what problem is after charging us $350 for diagnostics! Just yesterday, I started car and it acts like it won't start initially and then starts. Oh and red battery light flashes when I start up and turn car off. HELP!

Sure sounds like the ignition switch. If your alternator was really bad your engine would continue to run until the battey went dead, then it would stop and it won't start.
Spending $350 for a diag is a rip off. The switch costs less than $100 and if you are even marginally handy with tools it will take less than an hour to replace it.
I've posted pics on how to do it, if you want to tackle it I can dig them up and repost them.

+1 on what Jim said. You could also unlock your instrument cluster

http://www.cardomain.com/ride/313409...-series/page-6

and read what the battery voltage with the car shut off & with the car running. With the car off the voltage should be 12+ volts and with the engine running (and the alternator working correctly) the voltage should be about 13.5+ volts. This would allow you to possibly eliminate the alternator as the source of your problems.

BTW-you have the high cluster on your 2002 540.

If you end up replacing the ignition switch, make sure to get the factory one. www.autohausaz.com has a good price, but to find it for an automatic transmission, you have to have the factory part number to search for it. It will not come up by searching via car make, year, and model.

Here is the part number: 61326901962

$77 with free shipping from Autohausaz.com The one they carry is from the original equipment supplier.