For interior plastic or vinyl, I use Meguiar's Natural Shine. It's water-based, not petroleum, so it doesn't have the high shine or the slipperiness. It can also be used on tire sidewalls, the black composite door handles on older MBs, and rubber/plastic trim for a low-gloss shine, so you get two products in one.

On the MB-Tex seats of my late C230 (W202), I used Lexol's Vinylex. It's much like the Meguiar's NS, but thinner, and flows better. I'd spray it on every few months and scrub it a bit with a microfiber towel. Got up a lot of dirt I didn't know was there.
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-- Paul W. (The Benzadmiral)
('03 Buick Park Avenue, charcoal/cream)
Formerly:
'97 C230, smoke silver/parchment; '86 420SEL, anthracite/light grey; '84 280CE (W123), dark blue/palomino

As mentioned, 303 is excellent. I also like finish kare "fk 108", it lasts a little longer than 303 and smells nice and fresh. FK has other products which are really good as well.

I use the 3M wet type silicone too and am very happy with the results. Inside and out. Really works well on an older dash and other aged and dry interior parts. It also will make sun baked rubber bumper parts and plastic door handles look like new again, and won't disappear in a week like some do. Also softens up old door seals and stuff. The 303 is my second choice and is easier to find the the 3M, which I always have to order online.

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H.D.B.
OBK #45
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'75 W115 240D, wife's dd, 197K
........'75 W115 300D, my dd, 245K
.. .. ..........'82 W126 300SD, the 'someday' project

Ditto....expensive stuff but well worth it!