No worries. When I had my hatch, I used to have the same problem. I used a zip tie, (several). One around the amp, one around the pigtail and another through the 2 zip ties and hogged the pigtail up. It worked but sometimes I would have to hog it tighter.
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You know, somebody actually complimented me on my driving today. They left a little note on the windshield, it said 'Parking Fine.'