Secondary Air Pump

My wife's 1999 Cabrio's check engine recently came on so we took it to the dealer.  He informed us the secondary air pump needs to be replaced (on the ticket, it said it does not shut off) and said it would cost $850 (of course, the warranty is expired)! 
 
Does anyone know about these air pumps, where they are located, how hard they are to change, etc?  My next door neighbor is a mechanic and said he'd look at changing it for me, but is not familiar with Volkswagens.

if it dosnt shut off its not bad
ther is a relay neer the brake booster that runs it try pulling it out and see if the pump stops

Thanks, I'll try that.

The air pump supplies the catayltic converter with air required to burn 'unburned fuel' as required. The EMS will turn it on as needed.