The antenna on the Navman sleeve doesn't work with the tomtom software wherever its placed !
The communication speed used in TT5 is too high for the sleeve to respond on is own antenna - i have a separate Holux antenna plugged into the side of the gps sleeve so TT5 basically uses this rather than the built in one (the antenna itself is placed on top of the dash). If i had a more recent gps sleeve, i think it'd work fine down there.
I know TTOne users with the 986, using a Brodt clip in a similar position, don't have to resort to an additional aerial so i think you'd be OK. However the Brodit clip in that position on the 996/986 prevents use od the cigarette lighter socket when TTOne is mounted directly onto the clip. The best thin i found when i had the 986 was a £5 vent clip from ebay, that hooked into the central air vents with a 'foot' that rested against the cupholder plate (but meant i couldn't use the cupholders that i'd never use anyway !)