Could you elaborate please?
Sub and 6x9's in the same box? Or all in the parcel shelf?
I shall cater the answer for both anyway lol
All in the parcel shelf:
Shouldn't have a problem, providing the parcel shelf is strong enough to support it all and the sub is a "free air" type.
All in a box:
If the sub does not have it's own chamber it will push and pull the 6x9's a fair bit and destroy them quite quickly. The sub must have it's own chamber, divided from regular coax etc speakers. They simply move too much air to be in the same chamber as speakers.
However, just touching on the parcel shelf idea again, there is much more airflow in the boot and it's not a sealed chamber, so a "free air" sub will perform fine there and will not rip the 6x9's apart, as the volume of air in the boot is free to move about the rest of the cab.
Side note for RTK
I've never been too worried about weight in the micra, because it does well as a work commuter even with a little 10" sub in a small custom enclosure i built. I try not to run the car on the limiter in the twisties because the motor has never been in great nick. And i recently purchased a twin turbo beast for to satisfy my need for speed anyway
Sadly, i may be giving up the micra soon, when i've sorted out a few things on the new car. Car will be sold with another motor though and PLENTY of spare parts. I just don't have time for the little micra any more