My 87 didn't have any real problems...

The speedo cable is the only thing that has the least length....so if you are sure there is slack, try giving it a little more force to come out. I remember mine took a little to "pop" it out of its nice fit. You won't break anything by pulling it with more force because it wont come out further than the speedo cable will. You should have a good 2-3 inches of room to unscrew the speedo before being able to pull the cluster out further.

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2016 Monsoon Gray Audi Allroad - 21k
2008 Black Mercedes E350 4Matic Sport - 131k
2014 Jeep Wranger Unlimited Sahara - 62k
2003 Gray Mercedes ML350 - 122k