I'm thinking about getting the Pioneer AVH-X5800BHS headunit for my 2005 Honda Pilot EX but I want to keep costs as low as possible, so can I use the stock subwoofer by tapping into the Sub+ and Sub- wires with some PosiTap connectors or will I have to buy a separate powered subwoofer. Is there an amplifier built in to the car or does the stock headunit amplify the signal inside the unit? I have heard that the subwoofer is 50W. I tried researching this but I got many mixed answers.
The AVH-X5800BHS has a 70 watt channel dedicated to a subwoofer. This is good into 2 ohms, so this should work very well on your Honda Pilot.
Refer to page 54 for wiring information for a subwoofer. Note that wiring depends on speaker impedance, so in your case you should see if the magnet is labeled, or measure DC resistance with a meter.
If you are unsure about the wiring, you should google or go to Crutchfield to locate a specific harness adapter for your head unit and specific vehicle. This will not only save a lot of time, it leaves the OEM stock harness intact, in case you ever change again or even change back to stock for resale.