At one time, more than a dozen auto plants dotted the sprawling city limits of Detroit. It was Motor City, the engine room of a nation that made for the rest of the world.

That changed. Detroit shrank. Factories closed. More than a million people have left the city, and nearly a decade ago, when the U.S. auto industry was hanging in the breeze, unemployment hit nearly 30%.

But now, sunlight is shining out of Detroit's long winter. For the first time in nearly 30 years, a ground-up auto plant is underway in Detroit, and it's only one piece — albeit a big one — of a $4.5 billion investment in new manufacturing announced earlier this year by a resurgent Fiat Chrysler.

The $1.6 billion Mack Avenue Assembly plant will be the home of the Jeep Grand Cherokee as well as an all-new three-row, full-size Jeep SUV. The company said that when the time comes, Mack will also produce plug-in hybrid versions and have the flexibility to build full battery electric models in the future.

"They're betting that what the world wants, Detroit's union workers can give them," said Detroit Local 58 Business Manager Brian Richard. "It's welcome news."

Construction at the Mack Avenue site began in May. At peak, later this year, between 500 and 600 members will be working two shifts to get the line ready for production by next spring, said Local 58 Assistant Business Manager Ric Preuss.

"It's an aggressive schedule, but I think it shows the company's optimism and faith in what we can do," Preuss said.

The transformation of the Mack site from engine production to truck assembly has cascading effects on five other FCA plants, four — Jefferson North, Warren Truck Assembly, Warren Stamping and Sterling Assembly — in or near Detroit, and the Dundee Engine Assembly plant, halfway between Detroit and Toledo.

Fiat Chrysler expects the investment will create more than 6,000 new jobs, nearly 5,000 at the Mack Avenue site alone, all under a United Auto Workers collective bargaining agreement, though about a dozen IBEW members from multiple contractors have historically been onsite under the National Maintenance Agreement.

Fiat Chrysler's investment in Detroit contrasts sharply with other auto manufacturers. For decades, when foreign companies built in the U.S., they built in the South, where unions are kept weak, wages are low and worker protections are limited.

Worse, GM recently announced it will shutter its Detroit-Hamtramck plant next year, and four others in the U.S. and Canada will follow.

Most of those plants had produced slow-selling sedans. Fiat Chrysler announced it was killing nearly all of its sedan production a year ago. But instead of closing shop, the company discontinued compact car production and retooled plants in Illinois, Ohio and Michigan to launch the new Jeep Wrangler, Ram 1500 and the new four-door pickup Jeep Gladiator.

The company's chief financial officer told investors in June that trucks and SUVs would account for 80% of revenue by 2022.

Mack Avenue started life a century ago as a stamping plant making parts for Plymouth, Ford and other companies. It has been owned by Chrysler since 1953, but it closed in the late '70s for 11 years. New Mack was built in the early '90s, first to produce the Dodge Viper, and then as an engine assembly plant, first making V-8 truck engines and then, for the last 15 years, the Pentastar six-cylinder engine.

Next, Fiat Chrysler will retool and modernize the Warren Truck Assembly Plant just across Detroit's northern border, the celebrated 8 Mile Road. When complete, the plant will produce the all-new Jeep Wagoneer and Grand Wagoneer. The $1.5 billion investment will be heavily biased towards retooling instead of construction, but Preuss said he expects several hundred Local 58 members will be needed for the conversion.

The Warren and Sterling stamping plants, also north of the Detroit line, will receive $245 million in upgrades, and continue making hoods, roofs, liftgates, floor pans and other parts for Jeep and Dodge vehicles.

Finally, the company will begin work on the Jefferson North plant, a few blocks away from Mack. The $900 million retool and modernization of the existing plant is expected to begin in late 2020. Jefferson North will build the Dodge Durango and next-generation Jeep Grand Cherokee. Fiat Chrysler said it expects to need 1,100 new workers when construction is complete.

The engine production lost from the conversion of Mack to assembly will not be sent out of the region. The Dundee Engine Plant, halfway between Detroit and Toledo, will get nearly $120 million in improvements, also built by members of Local 58, and begin turning out the engines for the Jeep Gladiator.

It's hopefully just the start of the auto industry's comeback in the Motor City, civic and union leaders say.