It may be that you are not going about it correctly. You need to partially latch the rear before closing down the front. (Push the latch into the hole until it just catches.) Then latch the front down. After the front is latched closed go back and pull the lever for the rear latch forward into the locked position. If you are trying to latch the front above the visors first, the rear won't lock into place. If this isn't the problem then you may have a problem with the fit of the top. Hope this fixes the problem for you.

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Don
'85 500SL (Euro) - 186,000 w/a complete restoration and engine rebuild at 154,000
'95 C280 - 174,000