Blank Nav screen

[astat1]

I took my 2002 to BMW to diagnose my blank Nav screen. My radio and cassette work. They diagosed my screen unit as bad ( wanted $2400 on a $2000 new option to fix). I walked out spending another $1500 on a variety of items that I didn't intend to ($575 for a thermostat). I bought a guaranteed functioning unit on my own (eBay) for $800 and had it installed (another $150).....same blank screen. They told me they thought it was the navigation unit in the trunk. The red light blinks on my Mk 3 unit. What do you think? Thanks