Owners whose cars have electrical problems are often advised by me and others to seriously consider a 4+ year old battery as the prime suspect behind the problem. I eat my own cooking when it comes to auto maintenance, here's my experience.
1/17/02 I preventively replaced my car's then six year old, original, new car battery with an identical M-B battery: $122, self installed.Four years and 1 week later nothing - even the dome light was dead! That's after only 13,650 miles, driven weekly, in the mild, SF Bay climate. This turkey of a battery must have been made at least a year before I bought it from the local M-B dealer!
So this time I bought an Exide Autolite 84 month Group 48 that was manufactured last month: $103, self installed.
Always consider an old battery a prime suspect.