I can only suggest:

1. Compare you and your brothers cars, side by side, tuned to the same TMC station

2. Monitor the ibus data and see if both cars are producing the same ibus data. Maybe the ibus receiver is faulty in the radio?

3. If both cars have ibus data, then swap the map discs from one car to the other. If the problem lies with a map disc, it is possible that it is scratched and that the scratch affects just the TMC database on the map disc.

4. Also try swapping nav computers from one car to the other, see if the problem lies with the nav computer itself. If so, look at the coding of both computers and compare at hex levels (ie: don't rely on the NavCoder display, look at the raw data). Let me know if the coding is different.

Good luck!