welcome to the forum...
the nav flickering, radio coming on and off - it is not a fuse... fuse, once blown, stays blown, and the circuit stays without power...
this could be a poor contact, a knicked, chaffed, broken wire, or a wire that intermittently grounds - you need to check the harness to see if you can visually spot the problem... once the harness from the nav is in your hands, you can knead it to see if the problem manifests itself... same goes for the radio harness...
in general, you need to get a better understanding of when the problem happens, and if the problem is related to any external force or event... you might have a defective nav unit, radio unit, both or neither...
and, of course, you can rip out the original system and stick in an aftermarket... I am not fan of such systems, but in a 12-14 year old car, the opinion of a "purist" is useless, the 20 year old technology is not up to par with today's gadgets...
many on this forum have done both - the troubleshooting and resolution of their problems, and retrofitting of aftermarket devices... your choice...
as far as "alpina"... not sure if the E53 had Alpina's involvement, other than 4.6iS engine influence, and possibly (even more remote possibility!) the 4.8iS influence...
Tell us more about your ride, always willing to learn something new... Maybe you have one of the limited "hockey editions" E53?
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