If a multi based character has stealth, does all parts need to be in hindering for the whole character to use stealth? And does that apply for the +1 for hindering when they don't have stealth, as well?

Yes. Only specific occupied squares that LoF are drawn to would benefit from Stealth or the Hindering Terrain defense bonus.

Ok, Thank you

A character never has to be in hindering terrain to benefit from Stealth. The line of fire drawn to the character must cross hindering terrain for Stealth to work.

This is true, but it would almost never benefit a Great Size character that can use Stealth.

EDIT: For some reason I thought the OP was referring to a Great Size Multi-Base character. All answers given are still correct though.

Adding to the Question... If the same Character (team base) was to move and 1 of its Squares are in hindering do they get the penelty for moving out of hindering if they start from a square of clear terrain?

If a multi-base character starts in even 1 square of hindering terrain, it will halve it's movement regardless of which square it starts counting movement from (barring any special movement abilities).

Page 17 of the rulebook:

Right I was just wondering if the multi-based character with stealth occupied only one square of hindering. Could you target the other square not in hindering with a ranged attack? And if so do they get the +1 for hindering?

Yes. You could.

No. They would not.