I believe there's a round plastic cap under the mirror. You can pop it out using a small flat screwdriver. This will expose a bolt which you can re-tighten to firm up the mirrors again. Incidentally, do you have the electric folding mirrors?

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In the garage
2008 Infiniti G37S Coupe (Traded)
2011 BMW X5 xDrive35i
2011 BMW X5 xDrive50i