Packard 56 is handy for sure but it has one fatal flaw: it doesn't protect the terminals from oxidation. So in the '70s Packard Electrical Division created the Weather Pack (get it? Weather-resistant Packard).
Though Weather Pack is a modular system it achieved its weather resistance by sacrificing some of its utility. For the most part, Weather Pack connectors are inline exclusively and don't connect to a specific component as Packard 56 can. Furthermore, the seal system prohibits more than one wire from meeting a terminal.
Weather Pack's relatively small surface-contact area limits constant current capacity to 20 amps, less than half the capacity of Packard 56. But the design is more than compensated for by availability: every parts store across the country has Weather Pack service parts. And they're almost insultingly inexpensive if you buy them right.
Metri Pack represents Packard's second-generation modular connector. The company completely revised the terminal shape to flat blades and rectangular slots and standardized it to a metric format, hence the Metri part in the name. Though every Metri Pack series has a sealed version, some variants trade the sealing component for a smaller package size. Anyway, not every connector needs the sealing capacity.
Like Packard 56, Metri Pack is available in more applications than inline. In fact pretty much all GM electrical components from the '90s onward feature Metri Pack connections.
Metri Pack isn't a connector design as much as a group of connectors based on terminal width. The 150-series connectors feature 1.5mm terminals and handle 14 amps; 280-series connectors feature 2.8mm terminals and handle 30 amps; 480-series connectors feature 4.8mm terminals and handle 42 amps; and 630-series connectors feature 6.3mm terminals and handle 46 amps.
The series uses a separate clip that retains the terminals. This TPA, or Terminal Position Assurance, retains the terminal at pretty much all costs. In fact, it works so well that some terminals lack the locking tangs.
The 280-series Metri Pack is incredibly versatile. Its footprint not so coincidentally matches the blade spacing of many universal automotive components like mini automotive fuses (APM/ATM), relays, and flasher modules, making it perfect for universal applications. In fact General Motors designs its modern fuse/relay panels to use 280-series terminals pretty much exclusively, something that makes them readily adaptable to other applications. Coolest of all, even-numbered 280 electrical connectors can be used as inline fuse holders—caps for two- and six-cavity female connects exist for this specific purpose.
Though the same fuse-holding application holds true with the 630 series, the series is far larger and less versatile. Because it and the 150 series use a slightly different terminal attachment we're going to devote another story entirely to them and some other slick connectors.