EF-29's are low end trigger shifter/brake lever assembly that came on my 2002 Trek 4500. The 8 speed rear shifter has ceased working properly. I can shift into larger sprockets fine from any position, but it won't shift into the smallest three sprockets. The trigger moves properly, but it doesn't index.

I've disassembled it (and discovered they almost certainly weren't intended to be disassembled outside the factory) and haven't found anything obviously wrong, but I think the indexing spring has gotten weak.

Is anyone intimately familiar with these shifters with further suggestions, or should I just buy a replacement?

I see replacement shifters (I'm looking at Sram Attacks...their Shimano compatible trigger shifters) come with cables already attached...am I correct that I can just thread these through the existing cable housings on my bike? Anything else to know about installing new shifters?

Thanks