I recall having a similar problem with a bricked NA 680, once the unit was cured BT was disable and would crash the unit if re-enabled. The solution was to upgraded both unit software [firmware] and BT software together which is maybe why you are finding that BT only works ok with the earlier firmware. For instance, you should have BT SW V3.40 when using main fw V4.90. Go to Tools>System>About and look at the versions. The BT module is in the flip-up satellite receiver and it may also be an inter-relationship there as well. Latest GPS sw version is 3.00.

Regarding the main firmware file which has been previously delivered by garmin in *.exe form. This is simply a self-extractor which you can extract yourself with 7zip for instance. If you have trouble, change the extension to .zip and even Windows Explorer will open it for viewing it in later versions of Windows. In the folder nuvi6xx is a Garmin folder which has GUPDATE.GCD in it. Copy that to the Garmin folder of your 660 overwriting any existing file and boot it. It is simply what the exe does, correctly places that gcd file in the unit.