Back light dim when start up then get brighter
Normal.
When the ignition is first turned on (but before the engine is started), the light is running just off the battery, which may not be fully charged.
When you engage the starter, this is the biggest power draw there is from the battery, causing the light to dim even more.
Once engine is running (if everything is working right), the alternator powers the light, as well as everything else.
In fact, if may/may not get brighter as you rev the engine.
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