Is it

>! legroom?

The bigger I am, the less I will hold. No one else would be so bold.

>! You sacrifice trunk space for legroom.

I get comfy in the car, as we travel real far.

>! Certainly makes things more comfortable.

I used to get used a whole lot. Now, nobody gives a squat.

>! Cars are getting smaller, also "squat" is somewhat of an allusion.

It's clear when I fail, you'll hear folks wail.

>! Yeah, especially when you slam your knees into the back of a set.

I was popular in the eighties. I sure miss my Mercedes.

>! I guess 80's Mercedes had a lot of legroom?

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I guess there is a dual possibility in 

>! the trunk.