I am looking into buying this 2006 S2000 but the seller has an aftermarket head unit installed by BestBuy. He said the audio control on the left side of the steering wheel no longer works. He still has the OEM head unit. Will this be reversible?

 

Yup, all the controls go through a plug that should already be behind the aftermarket HU (not plugged in though). The OEM HU has the correct place for the plug to attach. Most aftermarket HU don't have a place to put the plug for the left-hand controls.

 

Yes, it's reversible. Just unplug the HU and plug the stock one back in. Pretty easy to DIY if you're so inclined:
https://www.s2ki.com/s2000/topic/404.../page__st__150

If you want to keep the aftermarket HU, you can also get a modifry DCI, which will make the dash controls work with the aftermaket HU.
http://www.modifry.com/products/dci/index.htm

 

Thanks!