Yes this sounds like an alternator issue. The 190E, and maybe the 300E also, has a control in the heated rear window to cater for just this situation. If there is not enough charge from the alternator, the heated rear window switch will flash its light when you turn it on, and disable itself. Try turning off your heated window manually (if it's on), and other high-drain electrical devices.

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190E's:
2.5-16v 1990 90,000m Astral Silver
2.0E 8v 1986 107,000m Black 2nd owner
http://www.maylane.demon.co.uk/190esmall.jpghttp://www.maylane.demon.co.uk/190esmall2.jpg