My 86 300E had exactly the same problem. The fuse kit is simply a band-aid, but it works for awhile. The fuse is blowing because of excessive power draw (duh) as the blower is not turning freely from age and wear. My eventual fix was to replace the blower motor ($150 on ebay) and install a 30 amp circuit breaker (from AutoZone, $7) outside of the fuse box. It has worked flawlessly.

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1996 SL320 97K miles
1996 C220 130k miles
1992 500SEL 170k miles <---sold
1986 300E 216k miles <---sold