I don't use these tools but here's a quick way to solve the time problem of when you activate your screensaver....
Do everything justintime says in his post. Then, to only have the screensaver (power dimmer) activate only when it's dark you could write a quick vbs script to get the current time and if it's greater than the time it gets dark in your area, then activate the screensaver. Here's some code.
Copy the above into notepad and save it as timecheck.vbs in your scripts folder or wherever.
Dim strRegKey, strRegValue
Dim strOn, curTime, cmpTime, activate, varOn
'This is where you set the cmpTime variable in this format TimeSerial(hh,mm,ss)
'hh is 24 hour format so 6:35 PM would be set as TimeSerial(18,35,00)
cmpTime = TimeSerial(18,35,00)
If curTime > cmpTime Then
varOn = "1"
varOn = "0"
strRegKey = "HKCU\Control Panel\Desktop\"
strRegValue = "ScreenSaveActive"
strOn = varOn
Set objShell = CreateObject("WScript.Shell")
objShell.RegWrite strRegKey & strRegValue, strOn, "REG_SZ"
Then go into Scheduled Tasks and create a schedule to run this script however often as you'd like maybe every 20 minutes. All the script does is activate the screensaver after a specific time.
It's not a perfect solution because there's that twilight period when you're driving but you can determine what's best.
Alternative to creating a Scheduled Task you could place this script in your "Startup" folder and run it once. But again, if you're driving for an hour and it goes from daylight to dark, it won't activate the screensaver. Anyway, just a suggestion.
 I just installed the powerdimmer.scr and it works fine. If you password on resume then the screensaver just goes straight to black. Good find on the powerdimmer!