91 Isuzu npr box truck glow plug Question?

It has the 4bd2 4Cylinder diesel engine with 20,000 miles on it and I don't think the glow plugs are working. It starts fine on a hot day but the colder it gets the harder it gets to start. It runs great once it starts. I here this truck has a glow plug timer in the electrical box near the drivers side wheel. If this timer is bad can I byp it and use a push on and a solenoid or relay to energize them. If so how long can I energize them without burning them up. I haven't had a chance to check anything yet as far as voltage to the plugs but I will tomorrow. I don't think they would all go bad at one time.