I have I've unplugged it from the factory antenna harness and the reception stayed the same.
There are a couple of things to check - I am familiar with Metra, but I think you are using a 3rd-party harness (unless that was just power and didn't include the antenna.
I also don't know the 2014 Corolla harness wiring.
Pioneer usually has among the best aftermarket FM reception - (oddly for reasons I can't explain OEM's tend to do better with general FM reception, voice command, etc.) Aftermarket does better with sound quality, expandability, iPod integration, display, etc. Pioneer should be AT LEAST very close if not better than OEM, though. HD reception is very spotty if you aren't in a major city downtown - but your unit is not HD capable.
What you verified is the radio works the same with or without the antenna connected, so it is the same as if you had the radio with no antenna connected.
Ideas:
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http://community.crutchfield.com/car_audio_and_video/f/27/t/26716.aspx Pin 8 is the antenna booster on some Toyota's - if you have the same plug type, see if connecting Pin 8 to the blue/white wire on the Pioneer helps.
- It's possible (maybe likely) that whatever adapter you are using to go from the Toyota wiring to the Motorola plug (
http://metraonline.com/part/40-LX11 - 40-LX11 if you were using Metra) is defective and/or it also needs it's blue wire connected to the Pioneer blue/white wire.
- Sometimes with some radios, the antenna works better if you either more fully or less fully insert the motorola plug into the radio. (I've never experienced it, and I've heard of it more with Clarion radios, but it's something to try.)
- Older Toyota's used a dual-diversity system with two antennas and the OEM radio would always select the strongest signal - but I don't think any modern ones do that.
- You can try using an underdash or even a standard antenna behind the dash and seeing how it works, but that would be a last result and shouldn't be required. -
http://www.sonicelectronix.com/cat_i519_universal-antennas.html?category_id=519&sort_order=price_asc - If you go this direction, I would buy locally that you can return if it doesn't help out or test out various models.
- Antenna boosters are available, but in my experience, they don't tend to help much - (and the one I tried destroyed my AM reception).
Let us know what you find out.