Been reading lots of cover letters recently. The job is for an English professor so the cover letters are quite important. I suspect they may play a different role when applying for a mechanic's job. For me, cover letters are often the determining factor since all the qualification issues have been resolved before I get the application. One page is usually better, but if you can get my attention enough to want to read the second page, you're doing well. I find that personalities often come across quite clearly in cover letters, helping with a decision. I think it helps to ask the question, 'What questions might the reader have upon reading this?" and try to answer them in the cover letter. So, if you describe a short period of employment, give a good reason to explain it (if you have one).
A mistake I have seen repeatedly, is a cover letter addressed to a different employer! Oooops

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13