I have an old tomtom 700 and had no joy with it at all in the c5. I managed to bag an external antenna thingy off ebay, and it works perfectly now. (which is lucky, as the speedo's just broken so it's all I have to tell me how fast I'm going!) The external aerial thing doesn't need to be outside th car, it seems to work ok even through the metal by virtue of just being a far better thing than the one inside the unit. They seem to go for £10-£15 ish complete with the bit that attaches to the unit.