People are:

1 – sits anywhere

2 – sits anywhere

3 – sits anywhere

4 – front only

5 – front only

6 – back only

Front seat has 18 possible combinations:

451 415 145

452 425 154

453 435 245

541 514 254

542 524 345

543 534 354

Back seat has 18 possible combinations:

612 126 216

613 162 261

621 136 236

623 163 263

631 316 361

632 326 362

Front and back seat combos are, I think,

18 times 18, or 324 possible seating arrangements.

I’m sure that there is some kind of formula to figure this kind of thing out, but I have no idea how to figure it!