Sounds like the new amp is drawing too much current, and the Nav is cutting out as a result.

Where did the power pick up (for the amp) come from?

It should come from the barttery direct and use the switched supply from the radio just to "switch" the amp on (not supply power to the amp).

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Sept 03, M3, Silver Grey. Retro-fits: Modded Mk4 DVD Drive, OEM Bluetooth, VR Control, Rainbow Speaker Upgrade, Sony SOT Amp, HK Sub, CCFL, Intravee II, NightMode.