I heated an exacto knife red hot, cut a horizontal slit in the cup...looked like this ...(/)...and it popped right in.

I tried it strelik way, couldn't get it to happen. Downside to my way is that on the hottest days (110's; plastic expands), when you hit a nasty hard bump, it might move (happed once or twice to me).

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Current:
1971 Mercedes Benz 250 (Euro Spec)
1972 Mercedes Benz 250 (US Spec)
Past:
1972 Mercedes Benz 280