Originally Posted by PropellerHead

In the E39's of the era with MKI-MKIV (MKIV=DVD-based), a factory navigation harness was different from the radio-only harness. I know of many folks who did the factory retrofit when the E39 was still in production. The navigation harness was a few $100 bucks then- probably $500, now- and had different plugs for the rear to the navi computer and radio as well as at the front to the new navi head unit. The only PnP that I know of is from the 4:3 navi to the 16:9 screen and the MKII-MKIV upgrades. I know bc I did them in my E39 about 9 years ago.

IIRC, those without DSP need to consider that all navi units are DSP. Those parts- DSP amp and harness- need to be considered when upgrading a non-navi E53.

Now all of this E39 reference may be nonsense, but I think it is beyond reasonable to consider that the head units and navi components are the same (with a slight pixel difference for viewing in the E53) through the concurrent production runs of E39 and E53. I think you'll find that you have to replace the factory radio harness with a factory navi harness for the E53 conversion as well. If the $1000 sale includes this harness, and the brackets for mounting the hardware in the front and the rear, you have a good deal. If it's just the front and rear units, expect to purchase around $5-600 of additional hardware to 'look' and 'feel' like an OEM install.